The present invention relates generally to the determination of the characteristic impedance of a load in an electronic circuit. More specifically, the invention concerns a method and apparatus to calculate complex load parameters independently of amplitude, impedance, or phase of the input signal source. The invention has particular application in a "smart" tuning scheme in which a transmitter is coupled to an antenna which has an impedance that is different than the characteristic impedance of the antenna.
It is well known in the field of RF signal transmission that the output impedance of an RF signal source should match the input impedance of its load. When the match is made, the efficiency of the power transfer between the source and the load is maximized. Theoretically, if the impedances are exactly matched, the load absorbs 100% of the signal transmitted from the source. On the other hand, when there is a difference between the load impedance and the impedance of the source, the load does not fully absorb the entire signal from the source and "reflects" a portion of the signal back toward the source.
In a conventional RF transmission system, a transmitter is connected to an antenna through a device normally referred to as an antenna coupler. The function of the coupler is to transform the impedance of the antenna to the desired characteristic impedance of the transmitter, normally referred to as Z.sub.0.
If all of the elements of such a transmission system remained constant, the coupler could be fixed and would not need to be changed. However, in practice, there are many factors which vary the impedance of the various elements in the transmission system, and thus, the impedance coupler must be changed. For example, the characteristics of antennas are known to change over time as the antennas endure ice loading, salt degradation, sag, and other elements of damage. Significantly, in the case of mobile RF transmission systems, the characteristic impedance of an antenna is known to vary considerably with changes in the natural surroundings of the antenna. Further, the impedance of the antenna and other elements in a transmission system is usually dependent upon the frequency of the signal being transmitted. Thus, mobile frequency hopping transmission systems encounter many changes in the characteristic impedance of the elements of the system for which it is necessary to change the operation of the coupler between the transmitter and its associated antenna.
Prior art systems used differing techniques to identify when the coupler needed it to be changed and to identify what changes needed to be made. For example, it was known in the prior art to measure the Voltage Standing Wave Ratio ("VSWR") of a transmitting system. If the VSWR exceeded a predetermined threshold, such as 2.5, prior art systems were known to attempt to alter the elements of the coupler to reduce the VSWR. Because the VSWR measures the ratio between the forward and reflected signals, it was considered a measure of the relative "goodness" of the impedance matching, a high VSWR indicating a relatively poor match and, accordingly, a relatively high reflected signal.
It was known in the prior art to use an array of inductors and/or capacitors which could be changed, either mechanically or electronically, so that the coupler could operate as a variable signal impedance transformer. For example, some early such systems used turbo servo driven variable capacitors and roller inductors to attain the necessary transformation. More modern systems use digitally switched inductors and capacitors banks controlled by a micro-processor based logical system. Such systems are illustrated in the U.S. Pat. No. 4,965,607 to Wilkins, et al. and in U.S. Pat. No. 4,799,066 to Deacon.
In previously known frequency hopping communication systems, it is generally desired to transmit only briefly, often for only milliseconds in duration, on a frequency, before the frequency is changed to the next frequency to be followed in the selected hopping scheme. Because the amount of time during which data is transmitted on any one hop is limited, it is often important that the tuning scheme which mates the transmitter to its antenna operate very quickly. Oftentimes, however, tuning schemes in the prior art which measure the VSWR of a signal being transmitted require a period of time longer than an individual hop to identify the appropriate antenna coupler configuration to provide impedance matching at the specific frequency. In such systems, it was known to measure the mismatch on a hop, to compute adjusted tuning values for the next hop at the same frequency and to store the adjusted tuning values in a memory device. Subsequently, when the hopping scheme was scheduled to transmit on a frequency which had previously been tested, the adjusted values would be utilized to set the tuning components, the signal transmitted, the mismatch computed and the tuning components revised and re-stored. In such systems, convergence of the determination of the tuning components proceeded in a trial and error fashion until an acceptable impedance mismatch occurred. Often the impedance mismatch was monitored and calculated on subsequent transmissions and if it once again exceeded an acceptable level, the iterative adjustment of the tuning components based upon the magnitude and direction of the impedance mismatch was again computed.
Prior art systems which utilize a measure of the impedance mismatch, such as the VSWR, had certain disadvantages in they sometimes would require the repeated or iterative attempts at changing tuning components while hunting for an acceptable mismatch level. While the mismatch was being reduced to acceptable levels, it is was known to transmit only training signals or predetermined signals so that information would not be lost while the mismatch was reduced. Such technique, while successful in many instances in reducing impedance mismatch to an acceptable level, did so at the cost of exposing the signal to interception by unfriendly persons interested in detection for jamming and intelligence gathering purposes. Typically, in many prior art systems, the training signals are sent at or near full power because the detection method was power dependent, or required a relatively large power to provide meaningful information. The additional transmissions for training purposes increase the likelihood of detection of the hopping scheme by personnel unfriendly to the user of the system.
It is, therefore, an object of the present invention to provide a novel system and method for determining the impedance of a load without a need for a multi-iteration trial and error tuning network adjustment.
It is still another object of the present invention to provide a novel system and method to determine the impedance of a load independently of the amplitude of the signal being transmitted.
It is still another object of the present invention to compute the impedance of a load independent of the phase or impedance of the input signal source.
These and many other purposes and advantages of the present invention will become apparent from a reading of the following description in view of the drawing figures.